In many lighting systems, dimming control is employed. The light output of a light source may be dimmed by a variety of different techniques, some of which depend on the type of light source which is employed. One type of dimmer is a phase-cut dimmer, for example a forward phase-cut dimmer.
FIG. 1 is a wiring diagram for a lighting system 100 with dimming control. Power is received from the public electricity grid as AC Mains voltage 110, and delivered to a load 130, for example one or more lighting units including one or more light sources under control of a phase-cut dimmer 120. In particular, phase-cut dimmer 120 may be a forward phase-cut dimmer which, for example, employs a TRIAC to cut AC Mains voltage 110 from being provided to load 130 for a Lighting system 100 may be installed in a residential or commercial building or facility.
FIG. 2A illustrates waveforms of a phase-cut dimming system when set at a first dimming level. In particular, FIG. 2A illustrates waveforms of lighting system 100 when phase-cut dimmer 120 is a forward phase-cut dimmer and is set to dim the light output of load 130 to dimmed by a relatively small amount. As illustrated in FIG. 2A, the input voltage V(input) (i.e., AC Mains) has a standard 120 VAC 60 Hz waveform according to electricity grid standards in the U.S.A. (in other countries, the voltage level and the frequency may be different—e.g., 230 VAC 50 Hz power, etc.). However, the voltage V(load) at load 130 does not follow the V(input) for the entire time of each half cycle of V(input), but instead is cutoff to be 0 V for an initial portion of each half cycle of the V(input). The point, or phase, in each half cycle at which the voltage V(load) is “cut back in” to follow V(input) so as to deliver power to load 130 is adjustable or controllable by a user via a user interface so as to control the amount of dimming applied to the light output by load 130. That is, the longer in each half cycle that the voltage V(load) is cut to zero, then the more the light output by load 130 will be dimmed. In the example shown in FIG. 2A, the power is phase-cut for less than 25% of each half cycle of V(input) so that a relatively large percentage of the available power is delivered to load 130, and the light output of load 130 is dimmed by a relatively small amount.
FIG. 2B illustrates waveforms of a phase-cut dimming system when set at a second dimming level which provides more dimming that is produced by the waveforms illustrated in FIG. 2A. In comparison to the example illustrated in FIG. 2A, the voltage V(load) at load 130 is cutoff to be 0 V for a substantially larger portion of each half cycle of the V(input)—more than 50% of each half cycle. Accordingly, much less power is delivered to load 130, and a much deeper level of dimming is achieved.
A variety of user interfaces exist for allowing a user to control the level of dimming of a lighting system. Examples of simple and well known user interfaces include sliders and rotary knobs. These kinds of user interfaces have one or more established “physical reference positions” defining minimum and/or maximum light output levels corresponding to deeper and shallower amounts of dimming, respectively. For example, a common vertical slider dimmer may allow a user to reduce the light output to a minimum level, or even turn off all light output, when the slider is in the lowest position, and to increase the light output to a maximum or full light level with little or no dimming when the slider is pushed up to the highest position. In that case, the lowest position of the slider is a physical stop that provides a physical reference position for the lowest light level, and the highest position of the slider is a physical stop that provides a physical reference position for the maximum light level. Similarly, a rotary knob user interface typically has a first stop at its most counterclockwise position that provides a physical reference position for the lowest light level, and a second stop at its most clockwise position that provides a physical reference position for the maximum light level.
The dimming circuit for use with such user interfaces can be relatively simple, with the slider or rotary knob directly adjusting the resistance value of a potentiometer or rheostat from a minimum value at one stop position to a maximum value at the other stop position, thereby setting a time constant for charging a capacitor to a triggering voltage for firing a TRIAC and setting the cut-in voltage of the phase-cut dimmer.
However, some types of user interfaces do not have one or more established “physical reference positions” defining minimum and/or maximum light output levels corresponding to deeper and shallower amounts of dimming, respectively. As used herein, the term “reference-free user interface” refers to a user interface which does not have physically defined positions corresponding to minimum and maximum dimming or light intensity settings. Accordingly, when a user interacts with a reference-free user interface, the user does not control the interface to indicate directly the level of lighting that the user wants to be provided, but rather the user controls the interface to indicate whether the present level of lighting should be increased or decreased. One example of a reference-free user interface is a rocker-type user interface.
FIG. 3 illustrates one example of a rocker-type user interface 300 for a dimmer for a lighting system. With rocker-type user interface 300, a user presses the rocker to be depressed at one end or side thereof (e.g., the topmost side 310) to increase the light output of from one or more lighting units (e.g., load 130), and presses the rocker to be depressed at the other end or side thereof (e.g., the bottommost side 320) to decrease the light output of the lighting unit(s). Since a rocker-type user interface does not include stops which provide physical reference positions to establish the minimum and maximum light output levels, some embodiments such as rocker-type user interface 300 shown in FIG. 3 include a visual indicator 330 (e.g., a series of lights or LEDs) which provides feedback to a user regarding the current light output setting of the dimmer.
Besides the rocker-type user interface, other reference-free user interfaces exist. For example a button-type user interface may include a pair buttons which a user may depress independently—one button indicating that the light level should be increased, and the other button indicating that the light level should be decreased.
In general, rocker-type user interfaces and other reference-free user interfaces which do not have physical reference positions are not able to directly adjust the resistance value of a potentiometer or rheostat, and therefore the existing dimming circuits for use with rocker-type user interfaces and similar user interfaces are generally more complicated and expensive than the dimming circuits described above for slider and rotary knob user interfaces.
FIG. 4 is a circuit diagram for a lighting system 400 with dimming control via a reference-free user interface 410 (e.g., a rocker-type user interface). Lighting system 400 includes a controller 420, a zero crossing detector (ZCD) 430, a DC voltage supply 440, a TRIAC firing circuit 450 and a TRIAC 460.
FIG. 4 actually illustrates via the dashed lines two different optional arrangements: a first arrangement where the dimming circuit is connected to the AC Mains line voltage, load 130, and the neutral wire; and a second arrangement where the dimming circuit is connected between the AC Mains line voltage and load 130, and the neutral wire is not connected to the dimming circuit. In the first case, ZCD 430 and DC voltage supply 440 are connected to the AC Mains line voltage and to the neutral wire and receive a voltage difference between the AC Mains line voltage ((V(input)) and the neutral wire (ground). In the second case, ZCD 430 and DC voltage supply 440 are connected to the AC Mains line voltage and to load 430 and receive a voltage difference between the AC Mains line voltage ((V(input)) and the load voltage (V)(load)).
In operation, ZCD detects zero crossings of the AC Mains line voltage and outputs to controller 420 a zero_crossing signal which indicates the timing of the zero crossings. DC supply 440 receives the AC Mains line voltage and supplies a DC voltage (e.g. 5 VDC) for operation of controller 420. A user may depress the rocker of rocker-type user Interface 410 to indicate that the user wants to increase, or decrease, the amount of light output by load 130. In response to a user depressing the rocker in either direction, user interface 410 supplies a dimming input control signal to controller 420. In response thereto, and considering the current light output level in response to previous inputs received via user interface 410, controller 420 determines a light or dimming level indicated by the user's interaction(s) with user interface 410, and a corresponding timing or phase with respect to the zero crossing time of the AC Mains line voltage to cut in the voltage V(load) to load 130 so as to provide the desired light or dimming level. Controller 420 outputs a dimming control signal to TRIAC firing circuit 450 which determines the timing when TRIAC firing circuit 450 triggers TRIAC 460 to conduction, thereby setting the timing in each half phase of the AC Mains line voltage (V(input)) when V(load) provides power to load 130.
However, the dimming circuit of lighting system 400 has some drawbacks. As noted above, the dimming circuit is somewhat complicated and more expensive than a potentiometer-based dimming circuit. Furthermore, when there is a lot of noise on the AC Mains line voltage, or when the frequency of the AC Mains line voltage is drifting, then the zero crossing time detected by ZCD 430 may be inaccurate and may vary from cycle to cycle, making it difficult or impossible for controller 420 to properly control the phase-cut dimming without some undesirable amount of time-dependent light variation or blinking. Additionally, in the case when the dimming circuit is connected between the AC Mains line voltage and the load and the neutral wire is not connected to the dimming circuit, the zero-crossing signal is not stable and is highly dependent on the load characteristic, for example during On/Off transitions, etc.
Thus, it would be desirable to provide a dimming circuit for a user interface which does not have established “physical reference positions” defining minimum and/or maximum light output levels which can overcome at least some of these drawbacks. In particular, it would be desirable to provide such a dimming circuit which can operate without a zero crossing detector.